Railroad car retarders are located beside the rails of the tracks. The retarder has elongated brake shoes, typically of steel, positioned on each side of the rails. The shoes move toward each other to pinch the wheels as a car moves through the retarder to slow its motion. The brake shoes are mounted on brake beams connected to levers operated by fluid cylinders.
The contact of the steel brake shoes with the steel car wheels produces a highly objectionable screeching noise as the car moves through the retarder. This noise may be hazardous to crew men working in the yard and offensive to adjacent residents. As a result, the problem of noise generation in railroad retarders has been a concern to industry for many years and, more recently, has attracted the attention of government.
Attempts have been made to reduce such noise to tolerable levels. In general, however, these efforts have achieved noise reduction at the expense of other necessary or desirable properties of the brake shoe, such as efficacy of retarding action and reduction in service life or have created other undesirable conditions. Such efforts have thus been unsatisfactory. These prior efforts have taken three general approaches: selection of materials for the brake shoes other than steel; altering the configuration of the brake shoes; or use of lubricants in the retarder.
U.S. Pat. Nos. 3,321,048; 4,003,451; and 3,716,114 showing the use of ductile iron, flake-graphite bearing iron, and asbestos, respectively, in the brakes shoes are typical of the first approach. While capable of reducing noise, due to the softness or lubricity of the materials, such shoes exhibited lessened service life due to increased wear and/or breakage as compared to conventional steel brake shoes. They therefore tended to be unsatisfactory from this standpoint.
U.S. Pat. No. 3,768,600 shows brake shoes in which the braking surface was altered to provide spaced, ribbed pads for noise reduction. However, the repeated shocks as the wheels moved from pad to pad were detrimental to the shoes and retarder mechanism and, in extreme cases, might damage the contents of the cars. In U.S. Pat. No. 4,003,451 the braking surface was varied along the length of the shoe. Analogous problems were encountered.
U.S. Pat. Nos. 3,838,646 and 3,874,298 show use of a water and oil emulsion as a lubricant to reduce noise. The spray of the lubricant in the air and/or its soaking in the ground, could be objectionable on safety and environmental grounds.